1. Field of the Invention
The present invention relates to a sheet stack ejecting apparatus for ejecting a sheet of paper on which an image is formed by an image forming apparatus such as a copier, printer, and the like, an image forming apparatus, and a sheet stack processing apparatus. More particularly, it relates to a sheet stack ejecting apparatus for ejecting a sheet stack, namely, a plurality of sheets of paper which are stacked, an image forming apparatus, and a sheet stack processing apparatus.
2. Description of Related Art
In various types of image forming apparatus generally, sheets of paper on which images have been formed are held and carried by rotating members such as an ejecting roller, an ejecting belt, and the like, and then, piled up on an ejected sheet holding tray arranged at downstream side of the rotating members with back ends of the sheets of paper being met with a back plate of the tray. For recent years, sheet ejection speed has become faster along with improvement of operation speed of respective sections in an image forming apparatus. Therefore, there has arisen a problem such that ejected sheets of paper on an ejected sheet holding tray are not piled up neatly. If sheet ejection is slowed down, disorderliness of sheet piling is reduced. However, operation efficiency as the entirety of the image forming apparatus goes down. As countermeasure of such a problem, there has been proposed a sheet ejecting apparatus that can lower rotation speed of an ejecting roller in the middle of ejections by detecting a back end of a sheet (for example, see Japanese Laid-open Patent Publication No. 11-255390). According to this type of sheet ejecting apparatus, sheet ejection speed slows down when a back end of a sheet passes through a nip of an ejecting roller. Since sheet ejection speed slows down when a sheet of paper is ejected to the ejected sheet holding tray, disorderliness of ejected sheets of paper is reduced.
On the other hand, there are some image forming apparatuses provided with a post-processing apparatus that conducts post-processing to sheets of paper on which an image has been formed. In such an image forming apparatus, plural of sheets of paper are piled up to make a sheet stack and ejected through post-processing such as stapling on the sheet stack, for example.
In case of ejecting a sheet stack also, ejection speed is so fast that a sheet stack hops out from an ejection slot vigorously and sometimes does not return to an backboard of an ejected sheet holding tray. Especially, in case of ejecting significant number of sheet stacks, sheet stacks ejected are piled up on an ejected sheet holding tray and sometimes sheet stacks hopping out too much are mounted on near the top of the pile. When such a situation arises, a back end of a sheet stack stays at the vicinity of an ejection slot and a front end of a next sheet stack bumps against the back end of the precedent sheet stack, which could cause an ejection error, and a paper jam.
However, the conventional sheet ejecting apparatus previously described is suitable for ejection operation to eject sheets one by one. In case ejection manner of the conventional apparatus is applied to ejection of sheet stacks, rotation speed of the ejecting roller must be changed in the middle of ejection operation. As a result, out of sheets belonging to a sheet stack, a top sheet and a bottom sheet in contact with rollers receive load. This aspect can cause disorderliness of a sheet stack, and wrinkles on sheets of paper, which are problematic.